I find fanatic spiritualist + pacifist OP. At least for my play style.
Transcendant republic gives +10% happiness and -20% food.
I rarely need more than a frontier clinic to provide enough food and a paradise dome (pacifist building). So all my other tiles can go to more useful buildings.
Fanatic spiritualist gives -30% ethics divergence. So ethics is pretty much never a concern.
This makes the liberate and vassalize strategy extremely potent.
Pacifist gives +5% happiness in peace time, which is the vast majority of my games.
Paradise dome from pacifist gives +10% happiness and +5% habitability.
There is the option to form enlightened or irenic monarchy as pacifist, giving access to royal gardens for extra +10% happiness. I never need this.
What my empire looks like:
My planets have frontier hospital and paradise dome for food, and they drown in food with those even on 25 tile planets. No hydroponics on top of this. All other buildings are used for energy, minerals or research.
90-100% happiness everywhere, usually 95-100%. There are exceptions among alien species in my empire. But it's exceptional to even see some alien pops here and there go below 75% happiness. It usually happens when I do quick integration (asap after war) on the edges of my empire. At that points it's irrelevant though.
Happiness: +10% from leader, +10% from government, +10% from paradise dome, +5% from peace, +5% from communal, +5% from Riggan spice, +10% from skill 5 governors in sectors.
On top of these come event related happiness and artist enclave festivals.
This means massive bonuses to everything.
Ethics divergence is -50%. Fanatic spiritualist -30%, Satramene gas -10%, Muutagan crystals -10%. From happiness: -16 to -20%, planetary capital -15%, conformists -20% (tend to use this on other species I integrate). If more is required, symbol of unity building for -10%.
Even across the galaxy my ethics divergence can stay negative.
The result is a play through with little conflict. It's very peaceful, turmoil free, and very diverse. I tend to end up with lots of species in my empire.
Expansion through war is easy by liberating. With all the -ethics divergence the vassal changes it's culture gradually and eventually you can easily integrate, ending up with aliens with high happiness.
I've not had horizon signals when playing as fanatic spiritualist/pacifist yet. That'd be an extra +10% happiness and -5% ethics divergence.